In prior art, so-called ICS systems (“infinity color-corrected systems”) are known, which consist essentially of an objective having chromatic residual errors and an infinite image distance, and a tube lens unit having a chromatically compensating effect. Thanks to such a combination, the intermediate image, despite the chromatic residual errors of the objective, is formed without chromatic aberrations, so that a very largely true-to-color rendition of microscopic specimen details is obtained throughout the image field.
A considerable disadvantage of the ICS systems known so far is the fact that the distance between the objective and the tube lens unit is variable within close limits only, so that these ICS systems are tied to given tube focal length and thus can only be use with instruments for which they have been conceived.
Frequently, however, it is desirable for the user, for cost reasons, to have a tube lens unit that can be used alternatingly with several optical instruments, each of which was originally conceived for a separate, fixed focal length.